Oracle SQL 不知道怎么优化

MERGE INTO TTPVMS200 A
    USING (
             SELECT b.COM_CD, b.TIC_LINK_ID, b.PROC_BASE_DTM, b.AREA_CD, DECODE(b.OP_CNGS_LVL_CD, '0', b.TRVL_SPD, b.OP_TRVL_SPD) TRVL_SPD,
                    DECODE(DECODE(b.OP_CNGS_LVL_CD, '0', b.TRVL_SPD, b.OP_TRVL_SPD), 0, 0,
                                            CASE WHEN (ROUND((b.TRVL_DIST / (DECODE(b.OP_CNGS_LVL_CD, '0', b.TRVL_SPD, b.OP_TRVL_SPD) * y.WGHTDIST))*y.WGHTTIME,0) < 99999)
                                            THEN ROUND((b.TRVL_DIST / (DECODE(b.OP_CNGS_LVL_CD, '0', b.TRVL_SPD, b.OP_TRVL_SPD) * y.WGHTDIST))*y.WGHTTIME,0)
                                            ELSE 99999
                                            END ) TRVL_TM,
                    b.TRVL_DIST,
                    DECODE(b.OP_CNGS_LVL_CD, '0', b.CNGS_LVL_CD, b.OP_CNGS_LVL_CD) CNGS_LVL_CD, b.PATT_TRVL_SPD, b.PATT_TRVL_TM, b.PATT_CNGS_LVL_CD, b.AREA_BASE_DTM
             FROM
                  TTPCMS200 b,
                  (
                     SELECT CASE WHEN x.SRC_DIST = x.TGT_DIST
                                 THEN 1
                                 ELSE ( SELECT WGHT_CONV
                                        FROM   TCMNCD011
                                        WHERE  UNIT_TYPE_CD = 'CMN05'
                                            AND    SRC_UNIT_CD = x.SRC_DIST
                                            AND    TGT_UNIT_CD = x.TGT_DIST )
                            END  WGHTDIST,
                            CASE WHEN x.SRC_TIME = x.TGT_TIME
                                THEN 1
                                ELSE ( SELECT WGHT_CONV
                                       FROM   TCMNCD011
                                       WHERE  UNIT_TYPE_CD = 'CMN07'
                                            AND    SRC_UNIT_CD = x.SRC_TIME
                                            AND    TGT_UNIT_CD = x.TGT_TIME )
                            END WGHTTIME
                     FROM
                            (
                                SELECT a.SPD_UNIT_CD, b.CODE_PARAM_VAL1 SRC_DIST, a.DIST_UNIT_CD TGT_DIST,
                                       b.CODE_PARAM_VAL2 SRC_TIME, a.TM_UNIT_CD TGT_TIME
                                FROM   TTMNMS001 a, TCMNCD002 b
                                WHERE  a.COM_CD = 'TIC0'
                                    AND    a.COM_DEF_CD = '00'
                                    AND    b.CATG_CD = 'CMN06'
                                    AND    a.SPD_UNIT_CD = b.COMN_CD
                            ) x
                   ) y                          
                WHERE
                        b.PROC_BASE_DTM = #procBaseDtm#
                        AND
                        b.AREA_CD = CASE WHEN #areaCd# IS NULL THEN b.AREA_CD ELSE #areaCd# END
          ) B
    ON (
            A.COM_CD = B.COM_CD
            AND
            A.TIC_LINK_ID = B.TIC_LINK_ID
    )
    WHEN NOT MATCHED THEN
            INSERT(
                A.TIC_LINK_ID, A.COM_CD, A.PROC_BASE_DTM, A.AREA_CD, A.TRVL_SPD,
                A.TRVL_TM, A.TRVL_DIST,
                A.CNGS_LVL_CD, A.PATT_TRVL_SPD, A.PATT_TRVL_TM, A.PATT_CNGS_LVL_CD, A.AREA_BASE_DTM
            )
            VALUES (
                B.TIC_LINK_ID, B.COM_CD, B.PROC_BASE_DTM, B.AREA_CD, B.TRVL_SPD,                
                B.TRVL_TM,          
                B.TRVL_DIST,
                B.CNGS_LVL_CD, B.PATT_TRVL_SPD, B.PATT_TRVL_TM, B.PATT_CNGS_LVL_CD, B.AREA_BASE_DTM
            )
    WHEN MATCHED THEN
         UPDATE SET
            A.PROC_BASE_DTM = B.PROC_BASE_DTM,
            A.AREA_CD  = B.AREA_CD,
            A.TRVL_SPD = B.TRVL_SPD,
            A.TRVL_TM = B.TRVL_TM,
            A.TRVL_DIST = B.TRVL_DIST,
            A.CNGS_LVL_CD = B.CNGS_LVL_CD,
            A.PATT_TRVL_SPD = B.PATT_TRVL_SPD,
            A.PATT_TRVL_TM = B.PATT_TRVL_TM,
            A.PATT_CNGS_LVL_CD = B.PATT_CNGS_LVL_CD,
            A.AREA_BASE_DTM = B.AREA_BASE_DTM 
    ]]>



不知道怎么优化提高效率，本人想把子查询转换成内连接或者用一种方式提高效率。
最好少用Select来完成结果。
请高手来帮忙！